In recent years, in the United States and Europe, more attention is increasingly paid to the prevention of health problems caused by discharge of various trace components such as mercury (Hg), lead (Pb) and iron (F) that are contained in the flue gas discharged from power plants, various factories and automobiles, in addition to nitrogen oxides (NOx), sulfur oxides (SOx) and the like contained in such flue gas. Thus, there is a movement to impose restrictions to reduce the discharge amount of these components to a very low level. Particularly, mercury discharged from coal-fired boilers is mostly released into the atmosphere in the form of mercury metal having a high vapor pressure, subsequently transformed into organomercury compounds, and ingested into human bodies mainly through fishes and shellfishes, to thereby produce harmful effects on human health. Furthermore, since mercury causes grave harms to the nervous system of growing infants, it has been reported that neurological abnormality cases approximating to 20% of the cases in American infants are suspected of being caused by mercury. As such, since mercury has serious adverse effects on humankind, attempts have been made in various aspects to reduce the discharge amount of mercury.
As a representative method of reducing mercury metal in exhaust gas, there is available a method of reducing NOx in exhaust gas with ammonia (NH3) using a NOx removal catalyst or an improved one thereof, simultaneously oxidizing highly volatile mercury metal into mercury in an oxidized form such as mercury chloride, and subsequently removing the mercury compound in an oxidized form together with combustion ashes or gypsum, using an electrostatic precipitator or a desulfurization apparatus, which is in the downstream area. In this case, a catalyst produced by adding an oxide of vanadium, tungsten or the like as an active component to titanium oxide is used (Patent Document 1).
These catalysts are conventionally used at a temperature of 350° C. to 400° C., which is the same temperature used in NOx removal reactions, in a NOx removal apparatus. However, when there are restrictions in the installation space such as in the case of previously installed boilers, installation of a NOx removal apparatus at the outlet of an electrostatic precipitator (EP) is being considered. Furthermore, since the reaction equilibrium of the oxidation of mercury metal into an oxidized form thereof, HgCl2, is dominant at lower temperatures, as the reaction temperature is lowered, it tends to be easier to obtain a high oxidation ratio. However, in a low temperature treatment of coal combustion exhaust gas containing sulfur dioxide (SO2) in a large amount, there is a problem that the catalyst component is sulfatized, or sulfur trioxide (SO3) which results from oxidation of SO2 is converted to sulfuric acid and accumulates in the catalyst pores, thereby causing the catalyst to be rapidly deteriorated. In order to prevent this, attempts have been made to install the catalyst in the downstream of the desulfurization apparatus, or to regenerate the catalyst by heating (Non-Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-53142    Non-Patent Document 1: G. Blysthe, B. Freeman, Bb. Lani, C. Miller, Mercury Oxidation Catalysts for Enhanced Control by Wet FGD, 2007 Air Quality Conference, Sep. 24-27, 2007